Very compact, but non-trimmable inductors may be realized in an integrated form within the upper metallization layers of a multichip module type D (MCM-D) substrate structure. Such inductors can provide inductance values between 1 and 100 nH within a 1 mm square footprint (using single or multilayer spiral structures), with self resonant frequencies between 20 GHz and 500 Mhz. The MCM-D ground plane may be removed from the area below the inductor in order to maximized the inductance and self resonant frequency. Quality factor in these MCM-D inductors is determined by the inductor resistance at low frequencies, while the peak quality factor is related to the nature and dielectric structure of the substrate employed. High resistivity silicon substrates with inductors defined in an aluminium-polyimide structure can provide quality factors between about 5 and 20, depending upon the inductor structure and inductance value. The peak quality factor occurs at a frequency between 0.25 and 0.5 of the self resonant frequency. Inductors on sapphire or other good dielectric substrates can achieve quality factors up to 30 in the same system, with the peak quality factor occurring at about 0.5 of the self resonant frequency. The effective cost of such inductors is directly related to the cost per unit area of the MCM-D structure, which is currently about 20 cents per square mm and will fall to 5 to 10 cents per square mm as production volumes rise.
United Kingdom Patent Application No. 9411107.7, corresponding to U.S. Ser. No. 08/463,311, filed Jun. 2, 1995, now abandoned, describes the construction of small, accurate discrete inductors that are to be assembled by flip chip solder bonding for use in multichip module (MCM), direct chip attach (DCA) and surface mount (SM) assemblies. These discrete inductors employ a spiral inductor structure defined in a low resistance copper metallization layer on the upper surface of a small dielectric chip. Metallic vias are included to make connections between the spiral structure on the upper surface and solder bond connections on the lower surface.
United Kingdom Patent Application No. 9413145.5, now U.S. Pat. No. 5,747,870, describes the use of a ferrite chip located over a spiral inductor defined in an MCM-D substrate that acts as a core to increase the specific inductance value or the quality factor of the inductor. The ferrite chip is aligned and supported over the inductor by flip chip solder bonding.